1. Field of the Invention
The present invention relates to apparatus which have electro-optic modulators for forming color images on a photosensitive surface in response to electrical signals.
2. Description of the Prior Art
Electro-optic materials are those whose optical properties change in accordance with the strength of an electric field established within them. These materials make possible an electrically controlled "electro-optic modulator." In this disclosure, the term modulator includes a device which changes the intensity of light in response to an applied electric signal. An electro-optic modulator includes a member formed of an electro-optic material which receives plane polarized (linear) light and which changes the state of polarization of such light in response to an established electric field. An analyzer receives light from the member and blocks that light whose plane of polarization has not changed (no established electric field) while transmitting light when its plane of polarization has been changed by an established electric field. By changing the electric field in the electro-optic member, light which passes through the analyzer is modulated.
One example of an electro-optic material used in modulators is lanthanum-doped lead zirconate titanate (PLZT). Although PLZT is a preferred electro-optic material, it will be recognized by those skilled in the art that other electro-optic materials can also be used to change the polarization of light.
Without an electric field being established, some compositions of PLZT are optically isotropic, while others exhibit a static birefringence. In either case, when an electric field is applied through a member made of PLZT, the PLZT crystal structure changes. This change in crystal structure causes a change in birefringence. An optic axis is thereby formed which is aligned parallel to the electric field lines. The optic axis is a direction and not just one particular line. The applied voltage produces the electric field. The intensity of light which passes through a modulator is a function of the applied voltage. The intensity of the field causes the polarization of light to rotate.
U.S. Pat. No. 4,229,095 discloses several embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optic, modulators to effect multi-color exposure of panchromatic imaging media. Two preferred approaches are disclosed for providing different color light for exposure. In one approach, separate radiation sources, such as separate elongated flash lamps with respective red, green and blue filters are energized successively by a controllable power supply. In the other approach, a rotating member having successive red, green and blue filter sectors is interposed between a single illumination source and the modulator array.
These approaches which use electro-optic modulators are quite suitable for many imaging applications; however, they have some undesirable aspects. For example, in the multi-lamp approach, there are certain limits as to how fast these sources can be turned on and off and for higher speed imaging applications, this can create a problem. The rotating filter approach avoids certain difficulties; however, it involves precise syncronization of the filter movement and thus involves the costs of precise servo-systems as well as potential operational difficulties in higher speed applications.
In another approach set forth in U.S. Pat. No. 4,366,499, an array of electro-optic modulators is formed of an electro-optic material. Different color filter layers are provided on different ones of the modulators. This approach requires techniques for producing such filters and for aligning them with the modulators.